Among the most devastating pathogens to infect man are bacteria that can replicate within endocytic vacuoles that restrict fusion with lysosomes. Residence in a non-degradative vacuole should severely diminish presentation of bacterial determinants on MHC class I and class II molecules. It is apparent, however, that cell-mediated adaptive immune responses play a pivotal role in host-defense against these infectious agents. The aim of this project is to resolve how an adaptive response is generated against a microbial pathogen that should have the capacity to restrict presentation of immunogenic epitopes by antigen presenting cells. The described approaches use the bacterial pathogen Legionella pneumophila as a biological tool to determine what effect inhibition of phagosome lysosome fusion has on the ability of an infected host to mount an antigen-specific immune response. Systems have been established to investigate host immune surveillance in vitro. Isogenic L. pneumophila strains have been constructed with defined loss-of-function mutations in genes that are essential for inhibition of phagosome lysosome fusion. Procedures to isolate bone marrow-derived macrophages and dendritic cells from permissive murine hosts have been established. Assays to measure presentation of L. pneumophila-specific MHC class I and class II-restricted epitopes by these professional antigen presenting cells have been developed. These reagents and assays will be used to systematically address whether residence in a non-degradative vacuole affords a pathogen protection from antigen processing and presentation following uptake into macrophages and dendritic cells. Experiments will be conducted to determine whether residence in a non-degradative vacuole affords a pathogen protection from antigen processing and presentation following uptake into macrophages and dendritic cells. Experiments will be conducted to determine whether findings obtained in vitro correlate with in vivo host responses. This will be done by measuring antigen-specific T cell responses in a murine model of infection using MHC class II tetramer labeling. These experiments will provide unique insight on whether residence in a non-degradative vacuole offers a degree of protection to a vacuolar pathogen from host immune surveillance. This information should help guide future research aimed at identifying the best antigens and delivery systems for stimulating protective T cell immunity against a variety of vacuolar pathogens.